It usually means that they don't know whether to believe you. Sometimes it's random.
At the CIC office they will study whether you've met your residency obligations. If they believe you have, they'll issue a PR card. If not, they'll investigate further, for example by interviewing you, and may begin proceedings to revoke your permanent residency.
See Sections 8.6 and 8.7 of Manual ENF 27 for an explanation.
http://www.cic.gc.ca/english/resources/manuals/